TLS certificate information retrieval

[Shikugawa]

Related with https://github.com/envoyproxy/envoy-wasm/pull/476. We require to retrieve TLS certificate information via wasm VM. As written, there are two approaches. In my opinion, we should add enum WasmMapType and include TlsPeerCertificate and TlsLocalCertificate. After that, we pass the type to proxy_get_map. Do you think about it? cc @PiotrSikora @jplevyak